Swain's team used this fortuitous orbit to study the planet's atmosphere using the star's light.
By measuring the drop in light as the planet moved in front of the star, and by seeing how the dimming changed among light of different wavelengths, the researchers were able to determine the chemical makeup of the planet's atmosphere.
The results showed signatures of methane and water vapor but not carbon monoxide, a gas that was expected to be abundant in the upper atmosphere.
The discovery of methane and water is especially interesting, because they are prebiotic molecules, which means they are capable of participating in the formation of amino acids, the building blocks of proteins produced by living cells.
Searching for Life
On Earth, the presence of methane is associated with life.
However, due to the extreme temperatures of HD 189733b, scientists know that the gas giant cannot support life as we know it.
Methane is thought to exist in the atmospheres of most gas giants. Our local gas giants—Jupiter, Saturn, Uranus, and Neptune—all show traces of methane.
(See an interactive map of the solar system.)
The method used by Swain's team could help in other future endeavors, said Adam Showman, a researcher from the University of Arizona.
Showman wrote an accompanying article on the new discovery for Nature but was not involved with the study.
"When transiting terrestrial [rocky] planets are detected in the future, this same type of method could potentially be used to infer the composition of the atmospheres of those planets," Showman said.
"Since life can alter the chemistry of an atmosphere, this would be a powerful way of searching for evidence of life."
Free Email News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES